The present invention is related to an ultra-low carbon steel composition. The present invention is also related to a process of production of an ultra low carbon bake hardenable steel having said composition. The present invention is also related to the end product of said process.
In the automobile industry there is a need for hot dip galvanized or galvannealed ultra-low carbon bake hardenable steel (also called ULC BH steel) having excellent dent resistance and very good paint appearance.
Several documents are describing such ULC BH products having either titanium (obtained by the so called Ti-route) or titanium-niobium (obtained by Ti/Nb-route).
More particularly, document EP-A-0064552 describes a method of producing a thin steel sheet having a high baking hardenability and adapted for drawing. The document describes a method comprising the steps of forming a molten steel having a composition containing 0.002-0.015% by weight of C; 0.04-1.5% of Mn; not more than 1.2% of Si; not more than 0.10% of P; 0.001-0.01% of N; 0.01-0.10% of Al, and Nb in an amount within the range (in %) from 2C to 8C+0.02 into a slab, hot rolling the slab, cold rolling the hot rolled sheet, subjecting the cold rolled sheet to a continuous annealing at a uniform temperature between 900xc2x0 C. and the Ac3 point, and cooling the annealed sheet to a temperature of not higher than 600xc2x0 C. at an average cooling rate of at least 1xc2x0 C. per second, preferably at least 10xc2x0 C. per second.
However drawbacks of this process are the high soaking temperature necessary to dissolve carbides and the fact that a high cooling rate after soaking is necessary to prevent reprecipitation of these carbides. Other disadvantages are the fact that beside the carbon content which must be controlled in a narrow range, also the Nb/C ratio in the steelmaking plant has to be controlled, and finally that, due to the use of Al for binding the N, high coiling temperatures are preferably used in order to prevent deterioration of mechanical and aging properties at the coil ends in case of continuously annealed steel. Higher coiling temperatures are disadvantageous for the pickling of the hot rolled steel before cold rolling.
Document JP-10280092 describes a hot dip galvanized steel sheet having minimal age deterioration in press formability and good baking finish hardenability. This steel has a composition comprising C, Si, Mn, P, S, Al, N, Ti, Nb, Fe and if necessary B, and is providing a metallic structure in which a specific volume percentage of iron carbide exists in the ferrite grain boundary. This metallic structure is formed by subjecting a slab of steel with the above composition to finish rolling at a temperature not lower than the Ar3 point, performing cold rolling at 65-95%, and then applying continuous hot dip galvanizing and temper rolling to the resultant steel sheet under respectively controlled conditions.
However, iron carbide precipitation in such kind of ULC steels was never detected in the as produced condition due to the very low amounts of carbon and the short times during which these low amounts can precipitate in a continuous annealing process. On the other hand, segregated atomic carbon in grain boundaries has long been physically known.
No BH0 values are mentioned. Also, according to the document, finishing rolling must be performed not lower than the Ar3 point which becomes more difficult in case of alloying with P and Si. No minimum Nb addition is specified in the abstract. Ti is added as a function of N and S-contents.
Document JP-5059443 describes a process of fabrication of a steel sheet having good formability which comprises the steps of adding Ti and Nb in relation with the C, N, S contents, while controlling carbonitride in an ultra-low carbon steel having a specific composition where Ti and Nb are combinedly added. This steel is hot-rolled at a finishing temperature (T2) higher than or equal to (Ar3xe2x88x92100)xc2x0 C., coiled at a temperature (T3) between 500 and 750xc2x0 C., and cold-rolled with a reduction of area higher or equal to 60%. Subsequently, this steel sheet is subjected to recrystallization annealing at 700-850xc2x0 C. by means of a continuous hot-dip galvanizing line having an in-line annealing furnace, and galvanizing is done in the course of cooling. By this method, a hot dip galvanized cold rolled steel sheet having required baking hardenability (BH characteristic) and formability can be obtained.
In particular, JP-5059443 requires that the Nb-content comply with the following condition: Nbxe2x89xa793/12[Cxe2x88x920.0015], wherein the Nb and C-contents are expressed in weight %. However, Nb addition as a function of carbon is an extra difficulty to realize in an industrial steelmaking plant.
Document EP-A-0816524 describes a cold-rolled steel sheet or a zinc or zinc alloy layer coated steel sheet containing 0.0010 to 0.01% of C and having a steel composition containing one or two kinds of 0.005 to 0.08% of Nb and 0.01 to 0.07% of Ti in the ranges given by specific relations. However, Nb and Ti are added specifically to have a minimum amount of fine NbC and/or TiC not less than 5 ppm, in order to get higher n-values. Moreover, said document gives explicitly a range for BH2 between 10 and 35 MPa, without mentioning BH0 values
Document JP05263185 describes a steel grade where the BH is in fact obtained by annealing in the two-phase (xcex1+xcex3) region followed by cooling which leads to a final acicular ferritic structure with a high dislocation density. A high Mn-content is needed in order to decrease the transformation temperatures. In order to have a good texture in the presence of a high Mn-content, free carbon during the recrystallization has to be avoided and is therefore being precipitated by Ti and Nb, before annealing is started. In the two-phase region some of these carbides are then dissolved providing free carbon. However, even with the large Mn-additions, the Ac1 temperatures are still high and annealing in the high temperature two-phase region is technologically a high cost-increasing factor.
Document JP04080323 describes a Ti-ULC BH steel which may contain 10-40 ppm Nb, without impairing the aimed properties. The claimed analysis also specifies a maximum N-content of 20 pppm, which is a high restriction for the steelmaking plant. However, prior research and industrial trial results have shown that with such Ti-ULC BH grades with a low  less than 40 ppm Nb addition, low yield strength occurs at the zinc bath temperature, which has a negative effect on the surface appearance of such steel sheets. The bad surface appearance of steel sheet obtained through the Ti-route is a consequence of small deformations, which are caused in the zinc bath and its immediate surroundings, by the high tensile stress in the zinc bath section and by the guiding rolls, which position the sheet between the air knives. In fact, the sum of the tensile stress generated by both the tensile forces applied to control the band behavior as well as the stress induced in the outer surface layers by bending of the sheet on the rolls in the zinc bath and by the imbricator rolls, may not exceed the yield strength of the material at the elevated temperatures of the zinc bath and its surroundings. The appearance is indeed increasingly bad at higher line tensile stresses and higher out of line imbricator roll positioning.
After stamping and before painting, this effect can be visualized on a Marciniak sample by way of transversal lines, even on sheets which have undergone the skinpass treatment and have been labeled as suitable for exposed parts. After the final painting of the surface, it exhibits an orangepeel-like appearance with high waviness. Due to this phenomenon, it can be expected that steels with a low yield strength (less than 220-240 MPa at room temperature) are most likely to suffer from this, which has indeed been verified in laboratory tests.
The present invention is related to an ultra-low carbon steel composition intended to be treated in a process comprising the steps from hot-rolling until hot-dip galvanizing or galvannealing and skinpass, said composition being characterized by the content of titanium, which is comprised between 3.42N and 3.42N+60 ppm for a fixed nitrogen content (N) and by the niobium content, which is comprised between 50 and 100 ppm, these contents being fixed so that no substantial precipitation of niobium carbides will occur during said process. More specifically, the present invention relates to an ultra-low carbon steel composition with the above characteristics, wherein no more than 2 ppm of carbon is bound in the form of Nb-carbides during said process
The composition of such an ultra-low carbon bake hardenable steel product is preferably characterized by
a C-content comprised between 15 ppm and 45 ppm,
a N-content comprised between 0 and 100 ppm, preferably between 0 and 40 ppm,
an Al-content comprised between 0 and 1000 ppm,
a P-content comprised between 0 and 800 ppm,
a B-content comprised between 0 and 20 ppm,
a Si-content comprised between 0 and 4000 ppm,
a Mn-content comprised between 500 and 7000 ppm,
a S-content comprised between 0 and 200 ppm, preferably comprised between 0 and 100 ppm,
the balance being substantially Fe and incidental impurities.
For a steel composition intended for galvanizing, the preferable carbon-content is comprised between 20 ppm and 25 ppm.
For a steel composition intended for galvannealing, the preferable carbon-content is comprised between 25 ppm and 30 ppm.
The present invention further relates to a process for producing an ultra-low carbon bake hardenable, galvanized or galvannealed steel product comprising the steps of,
preparing a composition wherein the titanium content is comprised between 3.42N and 3.42N+60 ppm, and the niobium content is comprised between 50 ppm and 100 ppm, these contents being fixed so that no substantial precipitation of niobium carbides will occur during the process,
if necessary, reheating said slab at a temperature (T1) higher than 1000xc2x0 C.,
performing a hot rolling having a finishing temperature (T2) higher than Ar3xe2x88x92100xc2x0 C. and preferably higher than Ar3xe2x88x9250xc2x0 C.,
performing a coiling at a temperature comprised between 500xc2x0 C. and 750xc2x0 C.,
performing a cold rolling in order to obtain a reduction higher than 60%,
annealing up to a maximum soaking temperature comprised between 780xc2x0 C. and 880xc2x0 C.,
performing a galvanizing or galvannealing step
performing a skinpass reduction comprised between 0.4% and 2%.
Reheating of the slab can be unnecessary if the casting is followed in line by the hot rolling facilities.
During the process, no substantial formation of TiC and NbC occurs, which is why a lower soaking temperature can be applied. Also, the use of Ti to bind the N is advantageous in that it solves the problem of high coiling temperatures. A maximum of 20 ppm N as described in one of the earlier mentioned documents is not necessary for the present invention which removes a difficulty for realization in the steelmaking plant.
Furthermore, the Nb-content is independent of the C-content, which solves the problem of the fixed Nb/C relation.
In order to achieve an increase of the yield strength at the zinc bath temperature, the necessary grain boundary modifications induced by the Nb are becoming effective at minimum 50 ppm Nb added. The presence of Nb ensures that the conventional yield strength Re0.2 at the zinc bath temperature (typically 460xc2x0 C.), of the steel sheet obtained by the process of the present invention, is a least 130 MPa. At 460xc2x0 C., microplasticity, for the steel obtained by the process of the present invention, starts at a stress level equal or above 70 MPa, which is a higher value than that of steels without Nb. Meanwhile, the yield strength at room temperature does not differ from the values obtained on these compared steels (having no Nb), which typically range from 160 MPa to 350 MPa after processing and temper rolling. This solves the problem of plastic deformation during processing in the zinc bath
Bake hardening values obtained on the final product are as follows:
Guaranteed BH0 en BH2 measured for a thickness lower than 1 mm, in the as skinpassed condition (measured according to the standard SEW094):
GI (galvanized):
BHo greater than 35 MPa, and  greater than 40 MPa at C greater than 20 ppm
BH2 greater than 40 MPa
GA (galvannealed):
BH0 greater than 20 MPa
BH2 greater than 30 MPa
The final product also exhibits an excellent dent resistance and a superior surface quality after stamping and painting, as a consequence of the absence of said plastic deformations occurring around the zinc bath section.